1. Field of the Invention
The present invention relates to a defective pixel detecting circuit for a solid state image pick-up device and image pick-up device including the same. More specifically, the present invention relates to a defective pixel detecting circuit of a solid state image pick-up device in, for example, a video camera or an electronic still camera which is capable of automatically detecting and correcting a defective pixel, and to an image pick-up device including the same.
2. Description of the Background Art
In an image pick-up device such as a video camera or an electronic still camera, a solid state image pick-up device such as a CCD (Charge Coupled Device) is used for converting an optical image to an image pick-up signal, generally. The solid state image pick-up device has two functions of photo-electric conversion and scanning. More specifically, the solid state image pick-up device includes photo-electric converting elements such as MOSs or photodiodes arranged two-dimensionally, forming a number of pixels. A charge image is formed in accordance with the intensity of light incident on each pixel, the charge images are scanned successively, and image pickup signals are taken in time sequentially.
Among the pixels of the solid state image pick-up device, there is possibly a so called defective pixel, which outputs a signal of an abnormal level without incident light. When there is such a defective pixel, quality of the pick-up image is degraded.
Such a defective pixel is generated not only as an initial defects, but it is also generated because of aging, as the solid state image pick-up device is used for a long period time.
The position of the defective pixel itself is fixed. Therefore, generally, the image pick-up signal from the defective pixel is corrected by storing the position thereof in advance.
More specifically, the defective pixel is detected at the time of delivery of the body of the image pick-up device and thereafter periodically, the position information of the defective element is stored in a storing device, and the image pick-up signal corresponding to the defective pixel is replaced by interpolation, using image pick-up signals from the pixels around the defective pixel. Such an interpolation is performed after the image pick-up device takes in the image signal and before processing the signal, by reading the position information of the defective pixel from the storing device and by interpolating the defective pixel by using the image pick-up signals one-dimensionally or two-dimensionally therearound.
Japanese Patent Laying-Open No. 5-260385 (H04N5/335), for example, discloses a method of detecting a defective pixel as follows. More specifically, the level of the image pick-up signal from each pixel of the CCD is compared with a prescribed threshold value with the light not incident on the CCD, and the position of the pixel outputting the image pick-up signal of an abnormal level, that means, higher than the threshold value, on the CCD, is stored in a register. When the pixel corresponding to the position data stored in the register does not continuously attain the abnormal level for a prescribed number of times (for example, ten times), the position data is erased from the register. Erroneous detection because of noise or the like can be prevented by this structure.
However, since it is necessary to include the white level to the black level of the CCD output in the dynamic range, the difference in level between the output of a normal pixel and of a defective pixel obtained at one photo-electric conversion is not so large, when outputs of the CCD with no incident light are considered. Therefore, if a defective pixel is to be detected by the structure of this prior art, setting of the threshold value is considerably difficult.
For example, if the threshold value is set too high, a defective pixel may be erroneously determined to be a normal pixel, and if the threshold value is set too low, the normal pixel may be erroneously recognized as a defective pixel.
Specially when a defective pixel cannot be recognized unless the output therefrom exceeds the threshold value continuously for ten times, for example, as in the above described prior art, a defective pixel may be erroneously determined to be normal if the output therefrom does not exceed the threshold value only once, and the defective pixel is overlooked.
Further, if the operation for detecting a defective pixel of the CCD is always performed at the time of power on of the video camera, that is, every normal image pick-up operation as in the prior art described above, it is possible to recognize generation of a defective pixel due to aging with high precision. However, it is necessary to monitor the image pick-up outputs from the CCD for a prescribed number of times in order to prevent erroneous detection, and it is necessary to operate the CCD and succeeding detecting circuitry such as an image memory for a certain time period for detection, which includes at least several frames. Namely, the signal processing inevitably consumes considerable power.
Now, aging of the CCD does not so frequently generate a defective pixel in general. Therefore, defective pixel detecting operation at every image pick-up operation is wasteful in view of power consumption. Further, recently, a battery having small capacity such as a dry battery is often used as power source for a video camera or an electronic still camera. In such a camera, frequent detecting operation means that much power is consumed for the detecting operation, and the time in which normal image pick-up operation is possible becomes shorter accordingly.